The invention addresses the problem of the diagnosis of the state of a load when a circuit driving the load is de-activated, that is, with its output at high impedance. The load may be in the short-circuited state (to ground, to the supply, or between its terminals). The circuit may also be in the open-circuit state (for example, because of a broken contact), or in a normal state, that is, connected and operating correctly. The diagnostic data may be supplied, for example, to a microcontroller which can implement recovery operations to ensure functioning in the event of a breakdown.
Obtaining the diagnostic data may be particularly advantageous for applications with supply voltages and ambient temperatures which are variable over a wide range, for example, in automotive applications. In this case, the diagnosis circuit if preferably able to withstand the maximum output voltage applicable to the load, for example, 50 V. Clearly, if the diagnosis circuit is produced in the form of an integrated circuit, components capable of withstanding high voltages occupy a larger area and therefore involve a higher cost. A reduction in the number of high-voltage components is therefore important for minimizing the area occupied by the circuit. Such a circuit is also desirably able to withstand the variations which take place in the characteristics of the electronic devices when the ambient temperature varies over a wide range, for example, from -40.degree. C. to 125.degree. C.
FIG. 1 is a circuit diagram showing a typical configuration in which the use of a diagnosis circuit may be necessary. One terminal of an electrical load LD is connected to an output OUT of a driver circuit DR. The driver circuit DR may be, for example, an amplifier, an output stage, or the like. It can be enabled or disabled (with the output OUT at high impedance) according to the value of an enabling logic input EN. When the driver circuit DR is enabled, a logic input IN selects the supply (positive or earth) to which the output OUT is connected. The other terminal TR of the load LD is connected to a supply, also to ground, or to another driver circuit (not shown). This diagram includes the possibilities of so-called "low side", "high side" and "bridge" driver circuits.
The basic layout used in diagnosis circuits according to the prior art is shown in FIG. 2. In FIG. 2 and the following drawings, parts and elements already described with reference to FIG. 1 have been attributed the same reference numerals and/or letters. The output of the driver circuit DR, that is, the terminal OUT is connected to two high-voltage polarizing resistors Rpu and Rpd, and to the high-voltage comparators CDT. When the enabling input EN of the driver circuit DR switches, disabling the circuit DR, the power transistors of the circuit DR are cut off and the voltage at the output OUT is brought to a value which depends upon the state of the output OUT, as described below:
if the load LD is correctly connected, the voltage at the output OUT is brought to a value close to that of the other terminal TR of the load LD, PA1 if the output OUT is short-circuited to ground, the voltage is brought to a value close to 0 V, PA1 if the output OUT is short-circuited to the supply Vdd, the voltage is brought to the value of the supply Vdd, and PA1 if the load LD is disconnected, that is, in an open circuit condition, the voltage at the output OUT is brought to a value determined by the voltage divider formed by the resistors Rpu and Rpd.
Discrimination between these conditions is carried out by means of the high-voltage comparators CDT. The situation in which the terminals of the load LD are short-circuited is distinguished from that in which the load LD is connected correctly by means of circuits, within the driver circuit DR, which can detect the presence of an overcurrent. Unfortunately, this approach has the disadvantage, however, that the resistors Rpu and Rpd and the comparators CDT have to withstand a high voltage, necessitating the use of large components. This approach according to the prior art also has the disadvantage of being very complex and consequently expensive.